1. Field of the Invention
The present invention relates to an indicating instrument and, more particularly, relates to an indicating instrument driven by a step motor and a method of operation thereof.
2. Description of Related Art
An indicating instrument which is driven by a two-phase-magnetic-field type step-motor has been used for a vehicle. The step motor of such indicating instrument has a pair of annular stators, each of which has a ring-shaped winding and a plurality of magnetic poles, and a permanent magnet rotor which has the same number of magnetic poles as each of the stator. The poles of one stator are disposed respectively between the poles of the other stator, and one of the windings is energized with a sine-wave current and the other is energized with cosine-wave current at the same time. A rotating magnetic field is generated by the stator changes as both current supplied to the windings change sinusoidally so that the permanent magnet rotor is driven in a direction and an indicator or a pointer of the indicating instrument moves together with the magnet rotor. The indicator is, usually, adjusted to stay at a zero position when it is manufactured.
If, however, a shock is applied to the indicating instrument before shipment, the magnet rotor may be rotated and the indicator may be separated remote from the zero position and may not be returned to the zero position.
The indicating instrument disclosed in Japanese Patent Laid Open Hei 6-38593 has a stopper located near the zero position, and a sine-wave current and cosine-wave current are supplied to the respective windings as zero-return current to form a backward-rotating-magnetic-field thereby to bring the magnet rotor and the indicator back to the zero position and rest on the stopper before the indicator instrument starts operation (forward rotation) so that the indicator can start moving from the zero position.
However, if the backward-rotating-field remains in a period longer than about a half cycle while the indicator and the magnet rotor are caught by the stopper, magnetic force of the rotating field moves opposite side of the indicator and drives the magnet rotor to separate the indicator from the stopper again. As a result, the indicator can not start from the zero position when it is operated for a normal use (forward rotation).
For example, if a half cycle (180 degree) of the sinusoidal-wave currents are supplied to the windings when the indicator is returned to the zero position (angle 0.degree. position) from an angle 30.degree. position, the indicator fluctuates between the zero position and repeatedly as shown in FIG. 12, and can not stay at the zero position as long as the sinusoidal wave currents are supplied to the windings.
U.S. Pat. No. 5,287,050 proposes zero-return-control of the indicator in which the zero-return current is cut when abutment of the indicator with the stop is detected from voltage induced across both terminals of the step motor.
However, switching operation of a driving circuit and a voltage detecting circuit are necessary with the result of a complicated structure.